The present application claims priority under 35 U.S.C xc2xa7119 based upon Swiss Patent Application No. 2001 0927/01 filed May 16, 2001.
The invention concerns a pressure transducer.
With the mounting of semiconductor chips, the semiconductor chips are bonded to the substrate with an epoxy adhesive. In doing so, it is necessary that a precisely determined amount of adhesive is applied to the substrate. Application of the adhesive takes place for example with a dispensing nozzle as is described in the European patent application EP 928637. The adhesive in the dispensing nozzle is pushed out in cycles by means of pressure pulses. The pressure pulse comprises an overpressure and an underpressure or vacuum phase. The vacuum phase always continues up to the next overpressure phase. It is necessary in order to prevent the adhesive from running out. The strength of the vacuum in this end phase is dependent on the characteristics of the adhesive and is given for each type of adhesive. Production of the vacuum is achieved for example with a device comprising a needle valve and a venturi nozzle connected downstream. The vacuum produced in this way is very non-linearly dependent on the position of the needle valve. In dynamic operation therefore, regulation of the vacuum is very difficult.
The object of the invention is to find a solution for regulating the vacuum of such a device.
A pressure transducer comprises a valve to which compressed air can be applied which has a valve block which works together with a valve seat, a motor for changing the position of the valve block and with it the pressure at the valve outlet, a sensor for measuring the pressure to be regulated and an electronic controller. The motor has a rotatable shaft the direction of rotation and speed of which are regulated by the electronic controller based on a set value and the signal delivered by the pressure sensor. The rotational movement of the shaft is transmitted into a longitudinal motion of the valve block via a coupling. The pressure transducer in accordance with the invention is characterised in that a spring is inserted between the shaft and the valve block. When the valve is closed, the spring is relieved or slightly pre-tensioned. The more the valve opens, the more strongly the spring is compressed: The spring exerts a force on the shaft which is dependent on the position of the valve block. It therefore puts a mechanical load on the motor. The characteristics of the motor are selected so that the mechanical load leads to the motor turning not with the speed given by the controller but with a lower speed. For the controller, this means a linearisation of the control characteristic. A needle valve is preferably used as the valve and the motor is preferably a DC motor.
In a preferred embodiment the pressure transducer comprises
a valve with an inlet for applying compressed air, the valve comprising a valve block and a valve seat,
a DC motor for adjusting the position of the valve block with regard to the valve seat,
means for transforming a rotating motion of a shaft of the motor into a longitudinal movement of the valve block,
a pressure sensor,
an electronic controller for regulating a direction of rotation and a speed of the motor based on a signal delivered by the pressure sensor and a set value for the pressure, and
a spring inserted between the shaft of the motor and the valve block for effecting a varying mechanical load on the motor in dependence on the position of the valve block, wherein an increasing mechanical load on the motor effects an increasing reduction in the speed of the motor in comparison with its speed in a no-load condition.
The pressure transducer can also be used for producing a vacuum in that a device for producing a vacuum, for example a venturi nozzle, is connected downstream of it. In this case, the sensor is preferably used for measuring the vacuum so that the regulated system not only comprises the valve but also the venturi nozzle.
The term pressure transducer means not only devices for producing a controlled overpressure but also devices for producing a controlled underpressure or vacuum.
In the following, an embodiment of the invention is explained in more detail based on the drawing.
In the following, embodiments of the invention are explained in more detail based on the drawing.